It is well known to provide a GPS receiver in which replica GPS satellite pseudorandom noise (PRN) code signals are continuously generated and correlated with received GPS signals in order to acquire them. Typically, as the replica codes are likely to have a different code phase to those of the received GPS signals and also a different frequency due to Doppler shift between the receiver and orbiting satellites, a two dimensional code frequency/phase sweep is employed whereby such a sweep will eventually result in the incoming PRN code having the same frequency and code phase as that of the locally generated replica. If detected, the code is acquired and tracked, and the pseudorange information may be retrieved from which the position of the receiver may be calculated using conventional navigation algorithms.
It is further known to provide a mobile cellular telephone incorporating such a GPS receiver for the purpose of enabling operators of cellular telephone networks to determine the location from which a call is made and, in particular, for an emergency call to the emergency services. Of course for an emergency call, it is desirable for the call location to be available as soon as possible, however, from a “cold start” where the GPS receiver does not have access to up to date ephemeris data or even worse from a “factory cold start” where the GPS receiver does not have an up to date almanac, the time to first fix (TTFF) can be anywhere between 30 seconds and 5 minutes.
In order to reduce the TTFF, a GPS receiver may be provided with base station assistance in order to acquire GPS signals more quickly. Such assistance may include the provision by the base station to the receiver of a precision carrier frequency reference signal for calibrating the local oscillator used in the GPS receiver; the data message for up to date satellite almanac and ephemeris data from which Doppler shift for satellites in view can be determined; and the current PRN code phase. With such assistance, it is possible to sweep only a narrowed range of frequencies and code phases in which the target PRN code is known to occupy, thereby reducing the number of code instances that need to be checked and thus reducing the time for code acquisition. Base station assistance is further described in U.S. Pat. Nos. 5,841,396 and 5,874,914 which are incorporated herein by reference.
A substantial reduction in the number of code instances that need to be checked enables an increase in the dwell time for each check without significantly affecting the overall time to acquisition. The benefit of this is that an increase in the dwell time increases the probability of acquiring weak GPS signals. For example, for a single code instance or dwell, correlation may occur over a period of 10 ms, equivalent to approximately 10 PRN code repetitions (C/A mode), or over a longer period such as 100 ms to 1 s worth of individual 10 ms correlation periods, incoherently summed as disclosed in U.S. Pat. No. 5,874,914. The upshot of the above is that it is becoming more feasible to consistently acquire and track GPS signals in order to obtain a position fix with a mobile GPS receiver located indoors where signal reception is often very weak. This is very important as many telephone calls to the emergency services made on mobile telephones are made indoors.
U.S. Pat. No. 5,905,460 to Odagiri et al. discloses a wrist watch comprising a GPS receiver and an alarm means which may take the form of a buzzer and is sounded in response to the watch being in a position where it can receive GPS signals (lines 31 to 33 of column 3) and when the GPS positioning calculations have been completed (lines 7 to 12 of column 3). It further disclosed in U.S. Pat. No. 5,905,460 that the alarm is sounded after the following chain of events: the “wave” receiving condition is determined to be not good, then the wearer of the watch is determined to be walking, then the wave receiving operation is stopped and finally either a predetermined time or distance walked elapses (FIGS. 11 and 12 with reference to line 117 of column 7 to line 17 of column 8). The purpose of such an alarm appears to be to reduce power consumption of the wrist watch (line 64 of column 1 to line 11 of column 2).